Ground Rules of Metabolism

1
Ground Rules of Metabolism

• Chapter 6

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6.1 What Is Energy?

• Capacity to do work
• Forms of energy
• Potential energy
• Kinetic energy
• Chemical energy

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What Can Cells Do with Energy?

• Energy inputs become coupled to energy-requiring
  processes
• Cells use energy for
• Chemical work
• Mechanical work
• Electrochemical work

4
First Law of Thermodynamics

• The total amount of energy in the universe
  remains constant
• Energy can undergo conversions from one form to
  another, but it cannot be created or destroyed

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One-Way Flow of Energy

• The sun is lifes primary energy source
• Producers trap energy from the sun and convert it
  into chemical bond energy
• All organisms use the energy stored in the bonds
  of organic compounds to do work

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Second Law of Thermodynamics

• No energy conversion is ever 100 percent
  efficient
• The total amount of energy is flowing from
  high-energy forms to forms lower in energy

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Entropy

• Measure of degree of disorder in a system
• The world of life can resist the flow toward
  maximum entropy only because it is resupplied
  with energy from the sun

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6.2 Energy Changes Cellular Work

• Energy changes in cells tend to run
  spontaneously in the direction that results in a
  decrease in usable energy

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Endergonic Reaction
glucose - a product with more energy
602 and 6H2O
Energy in
energy-poor starting substances
6
12
Figure 6.5a,bPage 100
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Exergonic Reaction
glucose - energy-rich starting substance

602
Energy out
6
6
products with less energy
Figure 6.5a,bPage 100
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Structure of ATP
nucleotide base (adenine)
three phosphate groups
sugar (ribose)
Figure 6.6bPage 101
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ATP Main Energy Carrier

• ATP couples energy inputs and outputs
• ATP/ADP cycle regenerates ATP

ATP
energy output
energy input
ADP Pi
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Electron Transfers

• Oxidation - lose electron
• Reduction - gain electron
• Central to the formation of ATP during
  photosynthesis and aerobic respiration

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6.3 Participants in Metabolic Pathways

• Energy Carriers
• Enzymes
• Cofactors

• Reactants
• Intermediates
• Products

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Degradative and Anabolic Pathways
large energy-rich molecules
ADP Pi
BIOSYNTHETIC PATHWAYS (ANABOLIC)
DEGRADATIVE PATHWAYS (CATABOLIC)
ATP
simple organic compounds
energy-poor products
ENERGY INPUT
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Types of Reaction Sequences
A
B
C
D
E
F
LINEAR PATHWAY
CYCLIC PATHWAY
G
K
J
I
BRANCHING PATHWAY
N
M
L
H
Figure 6.8Page 102
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Which Way Will a Reaction Run?

• Nearly all chemical reactions are reversible
• Direction reaction runs depends upon
• Energy content of participants
• Reactant-to-product ratio

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Chemical Equilibrium
RELATIVE CONCENTRATION OF PRODUCT
RELATIVE CONCENTRATION OF REACTANT
HIGHLY SPONTANEOUS
EQUILIBRIUM
HIGHLY SPONTANEOUS
Figure 6.9Page 103
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Chemical Equilibrium

• Energy in the reactants equals that in the
  products
• Product and reactant molecules usually differ in
  energy content
• Therefore, at equilibrium, the amount of reactant
  almost never equals the amount of product

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No Vanishing Atoms

• Law of conservation of mass
• Reactions rearrange atoms, but they never destroy
  them
• As many atoms of each element in all the products
  as there were in all the reactants

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6.4 Electron Transfer Chains

• Arrangement of enzymes, coenzymes, at cell
  membrane
• As one molecule is oxidized, next is reduced
• Create H concentration and electric gradients
  that are used for making ATP

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6.5 Enzyme Structure and Function

• Enzymes are catalytic molecules
• They speed the rate at which reactions approach
  equilibrium

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Four Features of Enzymes

• 1) Enzymes do not make anything happen that could
  not happen on its own. They just make it happen
  much faster.
• 2) Reactions do not alter or use up enzyme
  molecules.

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Four Features of Enzymes

3) The same enzyme usually works for both the
forward and reverse reactions. 4) Each type of
enzyme recognizes and binds to only certain
substrates.
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Activation Energy

• For a reaction to occur, an energy barrier must
  be surmounted
• Enzymes make the energy barrier smaller

activation energy without enzyme
starting substance
activation energy with enzyme
energy released by the reaction
products
Figure 6.12aPage 105
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Activation Energy

• Used to
• Align reactive chemical groups
• Briefly destabilize electric charges
• Rearrange, create, and break bonds

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6.6 Transition State

• Point when a reaction can easily run in either
  direction, to product or back to a reactant
• Substrate is bound most tightly to an enzyme in
  this state

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Mechanisms of Bringing about Transition State

• Helping substrates get together
• Orienting substrates in positions favoring
  reaction
• Shutting out water
• Inducing changes in enzyme shape

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Factors Influencing Enzyme Activity

• Coenzymes and cofactors
• Allosteric regulators
• Temperature
• pH
• Salt concentration

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6.7 How Is Enzyme Activity Controlled?

• Allosteric Activation
• Allosteric Inhibition
• Feedback Inhibition

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Allosteric Activation
enzyme active site
allosteric activator
vacant allosteric binding site
active site cannot bind substrate
active site altered, can bind substrate
Figure 6.15aPage 108
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Allosteric Inhibition
allosteric inhibitor
allosteric binding site vacant active site can
bind substrate
active site altered, cant bind substrate
Figure 6.15bPage 108
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Feedback Inhibition

enzyme 2
enzyme 3
enzyme 4
enzyme 5
A cellular change, caused by a specific
activity, shuts down the activity that brought
it about
enzyme 1
END PRODUCT (tryptophan)
SUBSTRATE
Figure 6.16Page 108
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Effect of Temperature

• Small increase in temperature increases molecular
  collisions, reaction rates
• High temperatures disrupt bonds and destroy the
  shape of active site

Figure 6.17bPage 109
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Effect of pH

Figure 6.17cPage 109